The protential use of many organic compounds, containing only carbon, hydrogen, nitrogen, and oxygen, as agents for the external localization of malignant neoplasms or for the visualization of specific organs has been hindered by the short half-lives of the gamma-emitting nuclides of these elements. However, the capability of the ORNL 86-inch cyclotron to produce multicurie quantities of carbon-11 activity makes it feasible to consider a wide variety of organic compounds labeled with carbon-11 (20.4 minute hal-life) as diagnostic agents in nuclear medicine. Tissue distribution data for analogous carbon-14 labeled compounds will be used as guidelines concerning the value of any potential carbon-11 compound. Rapid synthesis, purification, administration, and uptake by the tissues or organs of interest will be required. The specific compounds that appear most promising at this point are 1-aminocyclopentanecarboxylic acid (ACPC), estradiol, and thymidine as tumor-scanning agents and tryptophan and ACPC as pancreas-scanning agents. However, we propose a continuing program of short-lived radionuclides, with initial emphasis on carbon-11. As other carbon-14 incorportation data become available, other compounds will be considered for labeling with carbon-11. We hope also eventually to consider compounds labeled with nitrogen-13. o